Nursery: don't act as a checkpoint when not running anything

Previously, a Nursery would checkpoint on exit, regardless of whether
it was running anything. This is a bit annoying, see python-trio#1457, so let's
change it to not checkpoint on exit. Now it won't be a checkpoint at
all if there's no tasks running inside.

However, this doesn't fully solve python-trio#1457, because we'll still inject a
cancel even if none of the child tasks raise Cancelled.

This seems to break
trio/_core/tests/test_asyncgen.py::test_last_minute_gc_edge_case
such that it no longer ever hits the edge case.

That seems like a pretty complicated test, so maybe @oremanj can say
better why exactly it's breaking with this change.

trio/_core/_run.py

@@ -925,14 +925,6 @@ def aborted(raise_cancel):
 
             self._parent_waiting_in_aexit = True
             await wait_task_rescheduled(aborted)
-        else:
-            # Nothing to wait for, so just execute a checkpoint -- but we
-            # still need to mix any exception (e.g. from an external
-            # cancellation) in with the rest of our exceptions.
-            try:
-                await checkpoint()
-            except BaseException as exc:
-                self._add_exc(exc)
 
         popped = self._parent_task._child_nurseries.pop()
         assert popped is self

trio/_core/tests/test_run.py

@@ -31,6 +31,7 @@
     wait_all_tasks_blocked,
     Sequencer,
     assert_checkpoints,
+    assert_no_checkpoints,
 )
 
 
@@ -1605,20 +1606,15 @@ async def test_trivial_yields():
         await _core.checkpoint_if_cancelled()
         await _core.cancel_shielded_checkpoint()
 
-    with assert_checkpoints():
+    with assert_no_checkpoints():
         async with _core.open_nursery():
             pass
 
     with _core.CancelScope() as cancel_scope:
         cancel_scope.cancel()
-        with pytest.raises(_core.MultiError) as excinfo:
+        with pytest.raises(KeyError):
             async with _core.open_nursery():
                 raise KeyError
-        assert len(excinfo.value.exceptions) == 2
-        assert {type(e) for e in excinfo.value.exceptions} == {
-            KeyError,
-            _core.Cancelled,
-        }
 
 
 async def test_nursery_start(autojump_clock):